Powerline monitoring has evolved a long way from the early years of a.c. power, when the electrical service was subject to very few regulatory standards. Monitoring the power line often involved nothing more than measuring it continuously with some form of a.c. voltmeter to verify that the voltage level was being 30 held within designated limits, typically 105 to 130 volts. There were frequent outages, lightning strikes and other excursions beyond the normal limits, however much of the a.c. power-operated equipment was relatively simple, robust and tolerant to transient power line anomalies, so power line voltage quality was not a matter of great concern at that time.
With the passage of time, as electrical power usage intensified and particularly with the advent of the computer age and the proliferation of complex electronic equipment of many different kinds, although the electrical power industry had made much progress overall in regulating line voltage and minimizing outages, power-line related disruptions and damage continued to increase rapidly, calling a great deal of new attention to power line quality, and a large new market developed for equipment both for the protection of the payloads as well as for monitoring and evaluating many previously neglected aspects of the quality of power line voltage.
It became imperative to determine some kind of practical limits to which a.c. power lines can be "kept clean" and to which high tech equipment can be "toughened" to survive and tolerate at least a designated unavoidable level of contamination in the power lines as being acceptable.
The technical detective work necessary to ensure compliance with regulations in this field is highly statistical in nature, even regarding such basics as calender time and geographic location, since weather and electrical storms are often strong and highly unpredictable factors. The available existing instrumentation for this kind of investigation has been found lacking in sophistication to meet the new demands of the ever-increasing problem case loads that are building up in this evolving field.
An example of a standard that has evolved in response to this problem, in 1983 the U.S. Department of Commerce published a guideline, summarizing the fundamentals of powering, grounding and protecting sensitive devices under the title "Federal Information Processing Standards (FIPS) No. 94". This was approved and adopted by CBEMA, the Computer and Business Equipment Manufacturers' Association.
FIG. 1 depicts a simplified version of the "CBEMA Curve" that was developed by CBEMA and other industry members in an effort to characterize power line voltage anomalies such as dips, swells, outages and other transient effects based on both amplitude and time duration of the anomaly. This standard is widely recognized and utilized by designers of power line operated electronic equipment.
The y axis is marked in a scale of amplitude expressed as a percentage of nominal or rated voltage and the y axis is marked in a scale of time duration of the transient disturbance. The "acceptable zone" is between curves 10A' and 10B'; thus a high amplitude transient excursion is "acceptable" only if the duration is short enough to remain in the acceptable zone under the curve, e.g. at point A, where an excursion reaching 250% of rated line voltage is "acceptable" because its duration is well under 100 microseconds. An excursion of this amplitude will enter the danger zone if its duration exceeds about 100 microseconds: e.g. point B is deep into the danger zone due its duration of about 4 milliseconds.
Applying the curves to power-operated equipment, if such is designed and/or protected in a manner to be able to tolerate the somewhat larger acceptable zone between curves 10A" and 10B", then the overlap between the two sets of curves represents a margin of safety.
The upper danger zone corresponds to transient surges above the rated voltage, while the lower danger zone corresponds to voltage dips or sags: the shape of the lower curves indicate that complete power outages to zero volts are acceptable up to a time duration of about 8 milliseconds.
Even with the best efforts of the electrical utility entities, there can be no guarantee that the line voltage will be kept within the acceptable zone at all times, due to unpredictable adverse field conditions, so this becomes a statistical issue. Based on a study of 1,200 site months of National Power Laboratory data, a typical location can expect to experience 289 disturbances per year falling outside the CBEMA boundaries.
The CBEMA curve represents a practical baseline standard to be met or exceeded in the original design of equipment; in most instances it should be technically feasible to apply the CBEMA curve in the specification and performance of line voltage tolerance testing during the development of new equipment, specially in the case of protective equipment such as line oltage conditioners and UPS units.
Despite all precautions, there are many problems and shortfalls in this field of a.c. power quality and equipment rotection at this time that may not be overcome in the foreseeable future, thus there is a large and ever-expanding need or equipment dedicated to the monitoring and evaluation of the quality of power line voltage with regard to anomalous excursions and dropouts.